LED is an efficient, low-cost and green solid state light emitting device, and it also has the advantages of safety, fast-response, stable performance, small volume, resistance to vibrancy and impact, long life, etc. Nowadays, LED has been used widely in the field of indicating lamp, signal lamp, mobile phone, etc. LED will likely go into the general lighting field to substitute for incandescence lamp and fluorescent lamp.
The simplest way to achieve white light is to combine LED chip with phosphor basing on color match principle (also called as “pc-LED”, or phosphor-converted LED). Full color display can be fulfilled by UV, purple or blue light chip coated with phosphor. There are three ways to obtain white pc-LED: first, coating yellow phosphor on blue LED chip; second, combining blue LED chip with green and red phosphors; and third, blending tri-color phosphors and UV or purple LED chip. On the other hand, green LED can be obtained by coating green phosphors on UV-LED. The efficiency of this phosphor-converted green LED is much higher than that of single InGaN green chip. Likewise, coating yellow phosphor on UV-LED can produce yellow LED, and etc. In a word, various colorful LED can be obtained by combining some excellent phosphors with UV, purple or blue light LED according to color match principle.
At present, the popular way to achieve white LED is coating yellow phosphor (Re1-rSmr)3(Al1-s,Gas)5O12:Ce (YAG:Ce, U.S. Pat. No. 5,998,925) (YAG is “yttrium aluminum garnet”) or (Tb1-x-y,Rex,Cey)3(Al,Ga)5O12 (TAG:Ce, U.S. Pat. No. 6,669,866) on InGaN blue chip. Both YAG:Ce and TAG:Ce present wide band emission when they are excited by blue light. But the luminescent efficiency needs further improvement. In addition, their emission peak can only be adjusted between 520 nm and 560 nm; this can not satisfy the requirements of LED with low relative color temperature and high color rendering index.
In U.S. Pat. No. 6,429,583, LED lamp was manufactured by combining blue LED chip with Ba2SiO4:Eu2+ phosphors. This phosphor emits green light with wavelength centered at 505 nm. However, because of the scarcity of red light, it is hard to achieve white light with high color rendering index.
U.S. Pat. No. 6,809,347 mentioned a kind of LED coated with Eu2+-doped silicate phosphor (2-x-y)SrO.x(Bau,Cav)O.(1-a-b-c-d)SiO2.aP2O5.bAl2O3.cB2O3.dGeO2:yEu2+ or (2-x-y)BaO.x(Sru,Cav)O.(1-a-b-c-d)SiO2-aP2O5.bAl2O3.cB2O3.dGeO2:yEu2+. It can be excited by UV or blue LED, and the emission can also be modified from olivine to yellow even to orange light by adjusting the variable of phosphor composition. In the case, white LED with low relative color temperature and high color rendering index can be achieved.
Additionally, U.S. Patent Application Publications 2006/0027781 and 2006/0028122 referred to A2SiO4:Eu,D phosphor, wherein A is at least one metal element selected from a group consisting of Ca, Sr, Ba, Mg, Zn, and Cd, and D as compensating agent, is at least one nonmetal element selected from a group consisting of halogen or P, S, N. Through adjusting the species or ratio of A, this phosphor can achieve 460-590 nm emission when excited by 280-490 nm light. Moreover, the introduction of D affects not only luminescent intensity but also emission peak. The effects are much obvious when D is F according to these documents.
Accordingly, due to its wider emission band, Eu2+ doped silicate phosphor is more applicable for LED with low color temperature, when compared with YAG:Ce (TAG:Ce). However, above silicate phosphors are just doped with single rare earth ion, and their luminescent intensity needs to be improved for its LED application. The inventors have found out that the luminescent intensity of silicate phosphor can be further improved by doping more rare earth ions.
The present invention improves the luminescent efficiency of silicate phosphor by double-doped and multi-doped activator ions. Furthermore, by adjusting the composition of matrix, the present silicon containing phosphor can combine well with different LED chip to make white or color light emitting devices.